Although radar has been known for only a relatively short time, it is inherently of immense value and has been intensively developed at great expense because of the ability to detect the presence, velocity, acceleration, range, height, azimuthal direction and direction of movement of a target, the nature and identity of which is often unknown. While interrogation and IFF equipment and procedures have been developed that provide identification of civil airliners and friendly military aircraft, this approach to identification of a radar target is inherently less than completely satisfactory because of the possibility of equipment malfunction and of the enemy developing the ability to deceptively negate IFF.
Because of the very obvious desirability of being able to determine the nature of the radar target from the radar return (i.e. the signal caused in the radar receiver by the target), great efforts have been, and are being, made to identify targets from the characteristics of the reflected signal. These reflected signal characteristics are often called the target signature.
One of the many difficulties in identifying targets by their radar signatures relates to the spectral smearing of the signature caused by frequency shifts of the carrier frequency fc. These frequency shifts occur most often due to doppler frequency changes resulting from, intentional or otherwise, variations of the velocity of the target in a direction radially to the radar.
In an effort to avoid the disadvantages of spectral smearing caused by varying carrier frequency, two methods have often been used in prior radar signature analysis.
One method analyzes only a very short time sample of the signature, thereby minimizing the adverse effect of the frequency variation of the carrier fc. This method, which provides for a study of the broad spectrum but only for a short time, has the disadvantage of limiting the spectral resolution and of discarding much signal information.
A second method filters out one component of the signature (with a tracking filter) and analyzes this one component in detail, i.e. such parameters as spectral width and amplitude statistics. This method provides for the study of a long time sample, but only over a narrow bandwidth, and has the disadvantage that the relation of the studied component to the rest of the signal is lost together with the related opportunities to obtain data by analysis of cross-spectral densities, covariances, multiple correlation functions, etc.
This invention seeks to avoid the disadvantages of prior methods of radar signature analysis by providing technique and circuitry wherein the carrier frequency of the input (signature) signal is tracked to obtain a reference signal. This reference signal is phase compared to the input signal, a process which in effect translates the carrier frequency to zero frequency and provides a stable (folded) spectrum which is independent of the frequency variations of the carrier and which can be analyzed over long time periods.
It is, therefore, an object of the invention to provide improved apparatus and an improved technique for analyzing radar return signals to identify detected targets.
Another object is to provide the techniques of, and apparatus for, stabilizing the modulation spectrum of a radar target signature by eliminating the effects of frequency variations of the carrier frequency.
Yet another object is to provide improved apparatus and techniques for studying the spectrum of a carrier modulated signal, the improvement resulting largely from apparatus and techniques which eliminate the effects of frequency variations of the carrier.
A still further object of the invention is to provide the techniques of, and circuitry for, improving radar target signature analysis by tracking the carrier frequency fc to produce a reference signal which is phase compared with the target signature to furnish a signature spectrum which is independent of variations in the carrier frequency.